The objective of the present research program is to elucidate the mechanism of phototransduction using single-step mutants of Drosophila. Over 80 mutations affecting the phototransduction process have been isolated. These fall into 12 complementation groups. Physiological and biochemical studies are being carried out on these mutants and on double mutants constructed in pair-wise combinations of some of the key mutations. From the results of these studies we hope to obtain clues to the actions of each gene, the sequence of gene actions, and interactions among the gene products. Some of the mutations are being tested to see if they are lesions in the structural gene(s) for opsin. The structural gene for opsin, when identified, will be used to probe the functional roles of rhodopsin in transduction.